Method for making ammonium metatungstate

ABSTRACT

A method for producing ammonium metatungstate from ammonium paratungstate includes preparing an ammonium paratungstate mixture including solid ammonium paratungstate and water. The mixture is contacted with a cation exchange material to lower a pH of the mixture to a pH range wherein metatungstate ion is stable and an insoluble tungstic acid is not formed. The pH of the mixture may be maintained in the pH range until the ammonium paratungstate mixture is converted into an ammonium metatungstate solution.

BACKGROUND OF THE TECHNOLOGY

1. Field of the Technology

The present disclosure is directed to a method involving the use ofcation exchange material for producing ammonium metatungstate. Accordingto certain embodiments of the method, a high purity crystalline ammoniummetatungstate may be formed from ammonium paratungstate.

2. Description of the Background of the Technology

Ammonium paratungstate (APT) and ammonium metatungstate (AMT) are usedas tungsten-containing raw materials in the catalyst industry. APT haslimited solubility in water and typically has been converted to a moresoluble peroxytungstate form for use in catalyst production. AMT isreadily soluble in water, but is more expensive than APT because it isproduced through difficult synthetic routes and with low yields. In theproduction of certain catalysts, aqueous solutions of the tungstatecompounds are prepared and adsorbed onto alumina or another solidsupport. The adsorbed tungstate is then converted to a catalyticallyactive tungsten, tungsten oxide, or tungsten carbide.

Several processes for producing AMT are disclosed in the prior art. U.S.Pat. No. 3,175,881 discloses one process for producing AMT from APT. APTis first calcined to drive off ammonia and water vapor. The calcinedmaterial is digested in 80° C. water, while preferably maintaining pH inthe range of about 3.5 to 4.0. Additional ammonia is driven off duringthe digesting process, and water is evaporated to concentrate thesolution. Insoluble tungsten oxide may be produced, and some APT remainsunconverted. This process produces AMT in a yield of about 75%.

U.S. Pat. No. 3,591,331 discloses a method for producing AMT from anaqueous ammonium tungstate (AT) solution using a solvent. The methodincludes contacting an AT solution with an organic extractant solution.The extractant solution includes di-2-ethylhexylphosphoric acid andwater insoluble hydrocarbon solvent, which extracts ammonium ions fromthe aqueous solution and lowers the pH to at least about 4.5. Theaqueous solution is removed from the organic solution and heated for aleast one hour. An essentially pure AMT reportedly is recovered, and themethod also reportedly avoids the formation of APT. U.S. Pat. No.3,857,928, however, reports that the organic phase used in the methodtends to be unstable, and that up to 10% or more of the total solidproduct may be insoluble phosphor-tungstates.

U.S. Pat. No. 3,857,928 discloses a process for producing crystallineAMT from AT solution using an ion exchange column that contains a weakacid cation exchange resin. The AT feed solution is passed through theion exchange resin until a pH of 3.5 is attained, at which point theeffluent is collected. The collected effluent, which includes somemetatungstate species, must be digested at a temperature of at leastabout 98° C. for at least about 5 hours to obtain substantially completeconversion of AT to AMT.

U.S. Pat. No. 3,857,929 discloses a batch process for producing AMT fromAT. The process involves introducing a strong acid cation exchange resincontaining sulphonic acid groups into an AT solution until a pH of about3.5 is reached. The resin is removed by filtration, and the filteredsolution is digested at about 98° C. for at least about 5 hours toobtain substantially complete conversion of AT to AMT. Crystallizationof AMT may be carried out in a conventional manner.

A method for producing AT, APT, AMT, or hydrated tungsten trioxide isdisclosed in U.S. Pat. No. 3,936,362. The process includes passingtungstate anions through an anion exchange membrane into an aqueoussolution containing ammonium cations under the driving force of anelectrical potential for a time sufficient to achieve a pH within therange at which the desired tungsten compound will form.

U.S. Pat. No. 3,956,474 discloses an AMT production technique in whichan AT solution is digested in the presence of silica. Digestion takesplace for at least 4 hours at a temperature of at least 98° C., followedby filtration to remove the silica from the AMT solution. Typically, theAMT solution includes about 0.4% by weight of silica after filtering.

U.S. Pat. No. 4,283,257 discloses a process for producing AMT from APTusing an inert liquid permeable medium and direct electric current. APTis introduced in the anode chamber of an electrolytic cell, which isseparated from the cathode chamber by an inert liquid permeable media.When direct electrical current is passed through the cell, ammonium ionsmigrate to the cathode compartment, resulting in an increasedconcentration of metatungstate in the anode chamber.

Digestion of calcined APT to form an AMT solution is disclosed in U.S.Pat. No. 4,504,461. The calcining process includes fluid bed roastingAPT at 275° C. to 305° C. to form a precursor of AMT. The AMT precursoris slurried and digested for 30 to 120 minutes at 70° C. to its boilingpoint. The pH of the resulting slurry is between 3.0 and 4.0, and may becontrolled in that range for consistent results.

U.S. Pat. No. 4,557,923 also discloses a method for producing AMT byroasting APT. APT is roasted at 275° C. to 300° C. for 10 to 20 hours.The roasted APT is added to a dilute solution of AMT having a pH of 3.6to 4.6. The roasted APT is added at a rate suitable to maintain the pHof the AMT solution at 3.6 to 4.2. After addition of all of the roastedAPT, the pH of the slurry is stabilized at 3.6 to 4.2. Afterdissolution, the resulting AMT solution is concentrated and insolublematerials are removed. Based on the WO₃ content of the starting APT, thereported yield of AMT may be at least about 97%.

U.S. Pat. No. 4,612,180 discloses a method involving first heating APTusing microwave radiation. A relatively constant volume of an aqueousslurry of the microwave-heated APT is heated at about 80° C. to about100° C. until slurry pH stabilizes at 5.8 to 6.8. The slurry pH isadjusted to 4.2 to 3.0 by addition of either dilute mineral acid or astrong acid cation exchange resin. The cation exchange resin is thenremoved from the pH-adjusted slurry, and the slurry is digested at 80°C. to 100° C. for 2 to 6 hours to form an AMT solution. The AMT solutionis concentrated by evaporation, and crystalline AMT is obtained in aconventional manner. Based on the WO₃ content of the starting APT, theyield of AMT reportedly is at least about 93%.

U.S. Pat. No. 4,612,182 discloses an AMT production technique includingheating APT at 100° C. to 250° C. for 1 to 8 hours to drive off at leastsome portion of ammonia and water from the APT. The heated APT isdigested in water at 80° C. to 100° C. for 2 hours to 6 hours atrelatively constant volume, while maintaining the pH at 4.2 to 3.0 byaddition of ammonia, if necessary. A solution including AMT and water isformed. The AMT solution is concentrated and filtered, and AMT iscrystallized. Based on the WO₃ content of the starting APT, the AMTyield of the technique is reportedly about 95%.

Despite the several existing techniques for producing AMT, a need hasexisted for a less expensive, high yield process for producing AMT.

SUMMARY

A method for preparing ammonium metatungstate includes preparing anammonium paratungstate mixture that includes solid ammoniumparatungstate and water. The ammonium paratungstate mixture is contactedwith a cation exchange material to lower the pH of the mixture to withina pH range at which a metatungstate ion is stable and an insolubletungstic acid is not formed. The pH is maintained within the pH rangefor a time period sufficient to convert substantially all of theammonium paratungstate mixture to an ammonium metatungstate solution.

In certain non-limiting embodiments of a method according to the presentdisclosure, water included in the ammonium paratungstate mixtureincludes at least one of deionized water, distilled water, and doubledistilled water. According to another non-limiting embodiment of themethod, ingredients including solid ammonium paratungstate and waterhaving a temperature of 80° C. to 99° C. are mixed to provide theammonium paratungstate mixture. According to yet another non-limiting ofthe method, the ammonium paratungstate mixture may be heated to atemperature of 85° C. to 95° C.

In certain non-limiting embodiments of the method, contacting theammonium paratungstate mixture with a cation exchange material includesmixing the cation exchange material with the ammonium paratungstatemixture. In certain embodiments, the cation exchange material andammonium paratungstate mixture are agitated.

In certain non-limiting embodiments, a pH range at which themetatungstate ion is stable and insoluble tungstic acid does not form is2.0 to 5.0. In other embodiments, the pH range is 3.0 to 4.0, or 3.0 to3.5.

According to an embodiment within the present disclosure, and withoutlimitation, the cation exchange material includes a cation exchangeresin. In a non-limiting embodiment, the cation exchange resin is astrong acid cation exchange resin. In other non-limiting embodiments,the strong acid cation exchange resin is at least one of a compoundincluding a sulfonic acid group and a compound including a phosphonicacid group.

According to certain non-limiting embodiments, the cation exchangematerial may be separated from the ammonium metatungstate solution.Separating the cation exchange material from the ammonium metatungstatesolution may include, for example, filtering the ammonium metatungstatesolution.

Also, according to certain non-limiting embodiments of a method of thepresent disclosure, the ammonium metatungstate solution may beconcentrated to form a concentrated ammonium metatungstate solution.Concentrating may include, for example, boiling the ammoniummetatungstate solution. Without limitation, in one embodiment, theconcentrated ammonium metatungstate solution is filtered to remove atleast a portion of insoluble residual particles from the concentratedammonium metatungstate solution.

In certain non-limiting embodiments, the concentrated ammoniummetatungstate solution is dried. In a non-limiting embodiment, dryingthe concentrated ammonium metatungstate solution involves spray dryingthe concentrated ammonium metatungstate solution.

According to another non-limiting aspect of the present disclosure, amethod for producing ammonium metatungstate from ammonium paratungstateincludes mixing ingredients including solid ammonium paratungstate andwater at a temperature of 80° C. to 99° C. to provide an ammoniumparatungstate mixture. In certain non-limiting embodiments of themethod, the water included in the ammonium paratungstate mixtureincludes at least one of deionized water, distilled water, and doubledistilled water. An amount of strong acid cation exchange resin iscombined with the ammonium paratungstate mixture to adjust the pH of themixture to 3 to 5 or, in certain embodiments, from 3.0 to 3.5. Theammonium paratungstate mixture is heated to 80° C. to 100° C. The pH ofthe ammonium paratungstate mixture is maintained within the range untilsubstantially all of the ammonium paratungstate mixture is converted toan ammonium metatungstate solution. The cation exchange resin isseparated from the ammonium metatungstate solution, and the ammoniummetatungstate solution is concentrated to form a concentrated solution.The concentrated solution is filtered to remove at least a portion ofinsoluble particles from the concentrated solution, and is dried toobtain solid ammonium metatungstate.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the apparatus and methods describedherein may be better understood by reference to the accompanyingdrawings in which:

FIG. 1 is a flow chart of a non-limiting, exemplary embodiment of amethod for producing AMT from APT according to the present disclosure;

FIG. 2 is a plot of the solubility of APT in water as a function oftemperature;

FIG. 3 is an SEM image of spray dried AMT powder produced according toembodiments disclosed herein;

FIG. 4 is an SEM image of as-received DOWEX® G-26 ion exchange resinbeads; and

FIG. 5 is an SEM image of DOWEX® G-26 ion resin beads after two monthsof use in one non-limiting embodiment of an AMT production methodaccording to the present disclosure.

The reader will appreciate the foregoing details, as well as others,upon considering the following detailed description of certainnon-limiting embodiments according to the present disclosure.

DETAILED DESCRIPTION OF CERTAIN NON-LIMITING EMBODIMENTS

In the present description of non-limiting embodiments, other than inthe operating examples or where otherwise indicated, all numbersexpressing quantities or characteristics are to be understood as beingmodified in all instances by the term “about”. Accordingly, unlessindicated to the contrary, any numerical parameters set forth in thefollowing description are approximations that may vary depending on thedesired properties one seeks to obtain in the methods for producingammonium metatungstate according to the present disclosure. At the veryleast, and not as an attempt to limit the application of the doctrine ofequivalents to the scope of the claims, each numerical parameter shouldat least be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques. All rangesdisclosed herein are to be considered inclusive of the end points.

Any patent, publication, or other disclosure material, in whole or inpart, that is said to be incorporated by reference herein isincorporated herein only to the extent that the incorporated materialdoes not conflict with existing definitions, statements, or otherdisclosure material set forth in this disclosure. As such, and to theextent necessary, the disclosure as set forth herein supersedes anyconflicting material incorporated herein by reference. Any material, orportion thereof, that is said to be incorporated by reference herein,but which conflicts with existing definitions, statements, or otherdisclosure material set forth herein is only incorporated to the extentthat no conflict arises between that incorporated material and theexisting disclosure material.

Referring to FIG. 1, a flow chart illustrating a non-limiting embodimentof a method 10 for producing ammonium metatungstate (AMT) from ammoniumparatungstate (APT) according to the present disclosure is presented.The method 10 includes a step 12 including preparing an ammoniumparatungstate mixture (APT mixture) comprising solid unheated APT andwater. According to one non-limiting embodiment of the method 10, thesolid ammonium paratungstate may be a high purity APT availablecommercially from Osram Sylvania, Towanda, Pa. USA. In certainnon-limiting embodiments of the method 10, the purity of the final AMTproduct of the method 10 may be improved by using one or more ofdeionized water, distilled water, and double distilled water in the APTmixture. In certain non-limiting embodiments, the APT solution mayinclude other liquid solvents, such as, for example, alcohols, or otherliquid reagents that promote dissolution of APT in the APT mixture andthat can be separated from the AMT product. In all cases, however, theliquid portion of the APT mixture is predominantly water. Given therelatively low solubility of APT in water, only a limited amount of APTdissolves into the water or the water-containing liquid portion of theAPT mixture, and a low concentration APT solution is formed.Accordingly, step 12 provides an APT mixture comprising solid APT and adilute liquid APT solution.

According to one non-limiting embodiment of the present method, toincrease the solubility of the solid APT the water or water-containingliquid used to produce the APT solution is heated prior to and/or afteradding the solid APT. FIG. 2 is a plot illustrating the solubility ofAPT in water as a function of temperature. According to one non-limitingembodiment of the present method, the water or water-containing liquidof the APT mixture is heated to 80° C. to 99° C. to increase thesolubility of APT in the liquid portion of the APT mixture. In othernon-limiting embodiments, the water or water-containing liquid of theAPT mixture is heated to 75° C. to 99° C., or to 90° C. to 95° C., toincrease the solubility of APT in the liquid portion of the APT mixture.In one non-limiting embodiment, water used to produce the APT solutionis heated to a temperature near but below the boiling point to enhanceAPT solubility in the water. An ordinarily skilled practitioner willrecognize that heating the water or water-containing liquid included inthe APT mixture to any temperature above ambient temperature will assistin dissolving APT within the liquid. It will be understood that heatingthe water or water-containing liquid of the APT mixture, whether beforeor after solid APT has been added to the water or water-containingliquid, is within the scope of certain embodiments of the methodaccording to the present disclosure. In another embodiment, the water orwater-containing liquid of the APT mixture may not be heated, althoughheating would tend to reduce the total time for conversion of the APT toAMT in the method.

Again referring to FIG. 1, method 10 further includes a step 14including contacting the APT mixture with a cation exchange material.The objective of step 14 is to lower a pH of the APT mixture to a pHrange at which a metatungstate ion is stable and at which an insolubletungstic acid does not form in the mixture. The cation exchangematerial, which may be, for example, a cation exchange resin, adsorbs orotherwise traps ammonium ions in the liquid portion of the APT mixtureand, in exchange, introduces hydrogen ions into the liquid portion. Thehydrogen ions hydrate to hydronium ions (H₃O⁺), which also is referredto as hydrated acid. This exchange and hydration process lowers the pHof the solution portion of the APT mixture, and the acidification of themixture may be nearly instantaneous when the cation exchange materialcontacts the mixture. Since the desired pH range for step 14 ispredetermined, the amount of a particular cation exchange resin thatshould be contacted with the APT mixture in step 14 can be calculatedbased on the total amount of APT originally included in the APT mixturein step 12. More specifically, a person having ordinary skill maycalculate the weight or volume of a specific cation exchange resinneeded in step 14 on the basis of a number of milliequivalents of theresin per milliequivalent of tungstic anhydride (WO₃) present in theAPT. More generally, those having ordinary skill may readily determinethe amount of a particular ion exchange material that should becontacted with a particular APT mixture in step 14 to adjust the pH ofthe mixture to a desired range.

In a non-limiting embodiment of a batch process according to the methodof the present disclosure, the step of contacting the APT mixture with acation exchange material (for example, step 14 in FIG. 1) can includeadding an amount of the cation exchange material to the APT mixture toproduce a reaction mixture. The cation exchange material can be addedincrementally to the APT mixture until the pH is adjusted within thedesired range, or a predetermined amount of cation exchange material canbe added to the APT mixture to adjust the mixture pH to the desired pH.In certain non-limiting embodiments, the cation exchange material can bemixed into the APT mixture while agitating the mixture. In onenon-limiting embodiment, agitating the APT mixture can occur through allor a part of the process of adding the cation exchanger material to theAPT mixture and/or after addition of the cation exchange material. Aperson skilled in the art will understand that agitation may beaccomplished using any suitable technique such as, for example,mechanical agitation (such as stirring) or ultrasonic agitation. Such aperson also will understand that agitation can decrease the time neededto dissolve solid APT in the liquid portion of the APT mixture, therebycorrespondingly reducing the time to convert the APT to AMT.

Because a nearly instantaneous exchange of ions can occur when cationexchange resin is added to the APT mixture, a skilled practitioner canincrementally adjust pH to the desired range by incremental addition ofthe resin, without inadvertently reducing pH below the desired range. Inany case, it is important to closely monitor the pH of the APT mixturewhen adding a cation exchange resin so that the pH is properly adjustedto the desired range. Further, it is important to properly calibrate andre-calibrate any pH electrodes or other pH measuring device that may beused to assess pH during the method. For example, pH electrodes used tomeasure pH during the AMT production process preferably are calibratedat least about every four in-service hours.

According to one non-limiting embodiment of the method 10 according tothe present disclosure, the combination of the APT mixture and thecation exchange material is heated to increase the rate of dissolutionof APT in the liquid portion of the APT mixture and, correspondingly,the rate of conversion of APT to AMT. In one non-limiting embodiment,the mixture including the cation exchange material produced in step 14is heated to 85° C. to 95° C. In other non-limiting embodiments, themixture including the cation exchange material is heated to 90° C. to100° C. or, alternatively, to a temperature within the range of 70° C.to just below the boiling point of the mixture. As noted, an ordinarilyskilled practitioner will understand that heating the mixture includingthe APT solution and the cation exchange material to a temperature aboveambient temperature will assist in dissolving APT and, correspondingly,will enhance the rate of conversion of APT to AMT. In certainembodiments of the method according to the present disclosure, themixture formed by combining the APT mixture and the cation exchangematerial is not heated, but in such embodiments the total time forconversion of the APT to AMT is greater than in embodiments of themethod in which such heating is used.

In certain non-limiting embodiments of the method according to thepresent disclosure, the cation exchange material is a cation exchangeresin. In certain of such non-limiting embodiments, the cation exchangeresin is a strong acid cation exchange resin. A cation exchange resingenerally includes an insoluble matrix material (a support structure)that is normally in the form of small (for example, 1-2 mm in diameter)beads composed of an organic polymer. The matrix material beads may havea highly developed structure of pores on the bead surface adapted toreadily trap and release ions. The trapping of an ion in a pore takesplace only with the simultaneous releasing of an ion from the pore, andfor that reason the process carried out on the bead surface has becomeknown as ion exchange. In certain non-limiting embodiments of the methodaccording to the present disclosure, the cation exchange resin is a weakacid cation exchange resin, although utilizing such a resin increasesthe time for conversion of an amount of APT to AMT relative to anidentical method utilizing a strong acid cation exchange resin.

In certain embodiments of strong cation exchange resins useful as ionexchange material in the present method, the resin is a compoundincluding multiple units having one or more sulfonic acid groups and/orone or more phosphonic acid groups which serve as cation exchangegroups. In one non-limiting embodiment, a cation exchange resin utilizedin a method according to the present disclosure comprises multiple unitsincluding sulfonic acid groups. According to one such non-limitingembodiment, the resin includes sulfonic acid groups in the form ofpoly(2-acrylamido-2-methyl-1-propanesulfonic acid). The cation exchangegroups may be bound to matrix materials in the form of, for example, aporous polymer, a gel polymer, or mixtures thereof. One particularnon-limiting embodiment of a cation exchange resin is a sulfonic acidfunctionalized styrene-divinylbenzene gel polymer. In anothernon-limiting embodiment of the method according to the presentdisclosure, DOWEX® G-26 resin, available from The Dow Chemical Co.,Midland, Mich. USA, is utilized as a strong cation exchange resin toadjust the pH of the APT mixture. DOWEX® G-26 resin includes a lowmetals content styrene-divinylbenzene bead gel matrix with sulfonic acidfunctional groups, has a mean particle size of 650±50 μm, and hydratesto a water content of about 45-52%.

Inorganic-based materials also may be used as cation exchange materialsin certain embodiments of a method for making AMT according to thepresent disclosure. In certain of such non-limiting embodiments, cationexchange materials may include zeolites, montmorillonite, clay, and soilhumus. In general, a cation exchange material useful in embodiments ofthe method according to the present disclosure is any cation exchangematerial know now or hereafter to a person having ordinary skill in theart and which can be used to suitably acidify an APT mixture accordingto the present method to a pH in a range in which APT converts to AMT.

Tungstate ions in acidic solution condense to isopolytungstates.Isopolytungstates are formed by adding hydronium ions (H₃O⁺) to thetungstate ion (WO₄ ²⁻). The various isopolytungstates have differingratios of hydronium ion to tungstate ion, and the particular ratio of anisopolytungstate is a function of pH. The ratio of hydronium ions totungstate ions in a tungstate ion solution increases as the solution pHdecreases. APT exhibits relatively low solubility in water at a pH near7. When a mixture of APT and water is contacted with a cation exchangematerial, the pH of the mixture (more precisely, the pH of the solutionportion of the mixture) is lowered. As the pH of the mixture is lowered,the pH enters a range within which the metatungstate ion is stable, andthe APT in solution converts to AMT. At too low a pH, insoluble tungsticacid precipitates, which results in a lower yield and contamination ofthe AMT product. Therefore, the pH of the APT mixture (more precisely,the pH of the liquid portion of the mixture, which includes dissolvedAPT) should be monitored carefully so that the pH is maintained in arange wherein metatungstate ion is stable and insoluble tungstic acid isnot formed. At lower pH values within this range, the rate of conversionof APT to AMT is higher, but lower pH values also risk precipitation ofinsoluble tungstic acid, thereby contaminating the AMT product andreducing AMT yield. In certain non-limiting embodiments of the methodaccording to the present disclosure, a pH range wherein themetatungstate ion is stable and insoluble tungstic acid is not formed isa pH range of 2.0 to 5.0. In other non-limiting embodiments, the pHrange wherein the metatungstate ion is stable and insoluble tungsticacid is not formed is one of 3.0 to 4.0, 3.0 to 3.5, 3.3 to 3.7, andgreater than 2.0 but less than 6.0. In one non-limiting embodiment ofthe AMT production method according to the present disclosure, aspecific pH value at which the metatungstate ion is stable and insolubletungstic acid is not formed is about 3.2. In a non-limiting embodiment,the pH is allowed to drift up to 3.8 to 3.9 by the time that the APT toAMT conversion is completed.

The reaction utilized in the method according to the present disclosureto convert APT to AMT is as follows:(NH₄)₁₀H₂W₁₂O₄₁.5H₂O_((aq))→(NH₄)₆H₂W₁₂O₄₀.3H₂O_((aq))+4NH₄ ⁺ _((aq)).

Referring again to FIG. 1, method 10 includes a step 16 of maintainingthe pH of the mixture within a pH range wherein the metatungstate ion isstable and insoluble tungstic acid is not formed. The pH may bemaintained in this way and the above reaction may be allowed to proceedfor a period of time sufficient to convert the APT mixture to an AMTsolution. In certain embodiments, the same or substantially the same pHrange as utilized in step 14 may be maintained during step 16. Also, ifdesired, the pH of the APT mixture may be adjusted upward withoutcontaminating the conversion of APT to AMT by adding ammonium hydroxide(NH₄OH) or APT to the reaction mixture. In addition, in certainnon-limiting embodiments, water may be added to the reaction mixture tomaintain the volume of the mixture as the APT dissolves and is convertedto AMT, and the AMT is extracted.

In certain non-limiting embodiments, the method according to the presentdisclosure is conducted as either a batch or a continuous process, andall or substantially all of the APT included in the reaction mixture isdissolved and converted to AMT. Because it is difficult to separatesolid APT from solid AMT, dissolving and converting all of the APTpresent in the APT mixture of methods according to the presentdisclosure enhances the purity and facilitates the purification of thefinal solid AMT product.

Again referring to FIG. 1, in the non-limiting embodiment of method 10the cation exchange material is separated from the AMT solution in step18 after the APT has been converted to AMT. A non-limiting embodiment ofthe present method includes filtering the AMT solution to separate allor a portion of the cation exchange material from the AMT solution.Other means suitable for separating AMT from an AMT solution known nowor hereafter to a person having ordinary skill in the art are within thescope of embodiments according to the present disclosure.

In certain other non-limiting embodiments of methods according to thepresent disclosure, the cation exchange material may be provided in aporous container that allows ammonium ions to flow into the containerand contact the ion exchange material, and hydrogen ions, or hydroniumions, to be freed from the ion exchange material and flow out of thecontainer. In certain of such non-limiting embodiments, separating thecation exchange resin from the AMT solution simply entails removing theporous container from the AMT solution.

With reference again to FIG. 1, the non-limiting embodiment of method 10further includes step 20 wherein the AMT solution is concentrated toform a concentrated AMT solution. One non-limiting embodiment of such amethod involves evaporating water from the AMT solution to provide aconcentrated AMT solution. One non-limiting embodiment of such a methodincludes evaporating water from the AMT solution by boiling the AMTsolution. The concentrated AMT solution may be cooled after evaporatingwater from the AMT solution. In one non-limiting embodiment of method10, step 20 involves concentrating the AMT solution until the density ofthe AMT solution is at least 1.8 g/mL. Other possible solution densitiesfor the concentrated AMT solution are within the scope of embodimentsdisclosed herein.

Again with reference to FIG. 1, a non-limiting embodiment of method 10includes a step 22 in which the concentrated AMT solution is filtered toremove at least a portion of any insoluble particles that may be presentin the concentrated AMT solution. In certain non-limiting embodimentsincluding the filtering step, filtering may include filtering outparticles having a size of, for example, 1.0 μm or greater, or 0.5 μm orgreater. In addition, other techniques suitable for separating insolubleparticles or colloids, including flocculation and gravity settling, areencompassed by the present disclosure.

In the method 10 shown in FIG. 1, solid AMT is produced in a step 24 bydrying the concentrated AMT solution. One non-limiting technique fordrying the concentrated AMT solution is spray drying the concentratedAMT solution. Spray drying and other techniques for drying solutions toprovide a solid product are well known to those having ordinary skill inthe art, and details of those techniques need not be provided herein.

After the cation exchange resin is filtered from the AMT solution, theresin can be regenerated for further use using conventional techniques.In embodiments not meant to be limiting, the cation exchange resin isregenerated by exposing the cation exchange resin to a mineral acid,such as, but not limited to, sulfuric acid or nitric acid. In onenon-limiting embodiment, the amount of mineral acid added to the resinis in excess of what is needed to replace all ammonium groups bound tothe used resin with hydrogen ions. In another non-limiting embodiment,the used cation exchange resin is regenerated with 100-300% excess H₂SO₄(75-200 mL concentrated H₂SO₄ solution/Kg of APT) for an exposure timeof at least an hour, but usually 24 hours. After exposure to the mineralacid in the foregoing non-limiting embodiments, the cation exchangematerial is washed with deionized, distilled, and/or double distilledwater until the water runoff attains a stable pH. The resin is thenreused.

It will be understood that in certain embodiments of the AMT productionmethod according to the present disclosure the APT mixture may becontacted with the ion exchange material in a continuous process, ratherthan in a batch process. One non-limiting embodiment of a continuousprocess includes flowing the APT mixture through a bed of a cationexchange resin or other suitable cation exchange material. The pH of theliquid portion of the APT mixture may be monitored at differentlocations along the bed. Optionally, means are provided at or adjacentone or more of the pH monitoring locations for adjusting and/ormaintaining the pH of the solution in those locations within a rangesuitable for converting APT to AMT as discussed herein. In particular,the pH may be adjusted at or adjacent the one or more pH monitoringlocations so as to be within a range wherein the metatungstate ion isstable and insoluble tungstic acid is not formed. In certain embodimentsof such a continuous process, the effluent from the ion exchangematerial bed may be a substantially pure AMT solution obtained from theAPT mixture after the APT is dissolved in the pH adjusted solution andconverted to AMT. All pH conditions disclosed hereinabove for abatch-type process apply equally to a continuous process.

One particular non-limiting embodiment of a method for producing AMTfrom APT according to the present disclosure includes heating water to atemperature of 80° C. to 99° C. The water may be, for example, one or amixture of deionized water, distilled water, and double distilled water.Solid APT is mixed into the heated water to form an APT mixtureincluding a liquid portion that is a dilute aqueous APT solution. Asufficient amount of strong acid cation exchange resin is added to theAPT mixture to adjust a pH of the liquid portion of the APT mixture to3.0 to 5.0. The combined cation exchange resin and APT mixture is heatedto 80° C. to 100° C., and the pH of the liquid portion is maintained at3.0 to 5.0, or more preferably at 3.0 to 3.5, until substantially all ofthe APT mixture is converted to an AMT solution. The cation exchangematerial is then separated from the AMT solution, and the solution isconcentrated to form a concentrated AMT solution. The concentrated AMTsolution is filtered to remove all or a portion of insoluble particlesand dried to obtain solid AMT.

The environmental impact and costs associated with methods forconverting APT to AMT according to the present disclosure may be reducedby regenerating and reusing cation exchange material and/or by recyclingwater used in the method. In one non-limiting embodiment according tothe present disclosure, for example, the method my involve a closed loopprocess flow cycle in which DOWEX® G-26 ion exchange resin iscontinuously regenerated and reused to convert APT to AMT.

Several examples illustrating certain non-limiting embodiments accordingto the present disclosure follow.

EXAMPLE 1

Before beginning the process to convert APT to AMT, a conventional pHprobe was calibrated using standard solutions at pH 4 and 7. 3000 mL ofdouble distilled water was heated to 90-95° C. Approximately 250 g ofAPT powder was added to the hot water, and the mixture was stirred usinga stir bar. DOWEX® G-26 cation exchange resin was added incrementally inportions of approximately 10 g until the pH of the liquid portion of theAPT/water mixture was between 3 and 4. The total volume of resinnecessary to reach a pH between 3 and 4 was approximately 675 mLresin/Kg APT. As the APT dissolved in the acidified water, it reacted toform AMT, thereby allowing more APT to dissolve in the solution anddrive the conversion of the APT toward completion. The dissolution andreaction of the APT initially occurred relatively quickly. As thereaction proceeded the reaction mixture changed from an opaque whiteslurry to a clear, slightly yellow or amber solution. When the solutionappeared clear, indicating that substantially all of the APT had beenconverted, additional APT was added in two 250 g increments until thesolution contained 750 g APT (250 g/L). The 750 g of APT reacted in theprocess (250 g initially+500 g added) was dissolved and converted to AMTin approximately 90 minutes.

EXAMPLE 2

AMT was prepared from solid APT and water as described in Example 1,except that a total of 1050 g of APT was incrementally dissolved in 3000mL of hot water (350 g/L). The time to convert all of the APT to AMT wasabout 6 hours. Although the complete conversion required 6 hours, the350 g/L concentration of the APT mixture was well below the solubilityof AMT in water, which is about 1635 g/L at 22° C.

EXAMPLE 3

The AMT solution prepared in Example 1 was filtered in an 18 cm diameterBüchner funnel to remove the cation exchange resin from the solution.The filtrate was then concentrated via evaporation at approximately 100°C. (boiling) until the solution density was 1.8 g/mL, thereby providinga concentrated AMT solution. The pH of the solution tends to become moreacidic during evaporation. In order to prevent the solution frombecoming overly acidic, and to prevent the precipitation of insolubletungstic oxide, ammonium hydroxide (NH₄OH) solution (10 mL ofconcentrated ammonium hydroxide diluted with double distilled water to100 mL) was added dropwise using a pipette to maintain the AMT solutionat a pH between 3 and 4. The ammonium hydroxide solution should not beadded too quickly, as sudden localized fluctuations in pH may causeprecipitation of insoluble tungstic oxide. The concentrated AMT solutionwas cooled. As necessary, the pH of the cooled concentrated AMT solutionwas adjusted to 3.5 with the ammonium hydroxide solution. Theconcentrated AMT solution was filtered to remove insoluble particles.The mass of insoluble particles was about 5 g. X-ray elemental analysisof the insoluble material showed that the insoluble material wascomposed mainly of tungsten and oxygen. The resulting solution had aclear amber appearance. The solution was stored until it was spraydried. Taking into account the loss of material due to handling and thematerial used for lab samples, the overall yield for the conversion ofAPT to AMT was approximately 99%.

While conducting the above-described examples, it was observed that itmay be important to prevent the cation exchange resin and AMT solutionfrom contacting metal containers, such as carbon steel containers.Cation exchange resin soaked with double distilled water and placed in acarbon steel container for several days was observed to absorb iron fromthe container. This contaminated the AMT product for several cycles,i.e., after regeneration and reuse of the cation exchange resin. Theregenerated resin previously in contact with carbon steel was found tohave a high iron content. Iron was not found in new resin that was notplaced in a carbon steel container. Resins in contact with stainlesssteel containers did not exhibit iron pickup.

While conducting the above-described examples, it also was observed thatthe pH measuring apparatus should be recalibrated frequently, such asbefore daily use and every four hours thereafter. Although the reactionconverting APT to AMT will still occur at pH outside those discussedherein, insoluble tungstic acid may precipitate in the reaction mixtureand thereby contaminate the AMT product. The conversion of APT to AMToccurs faster at lower pH, and this requires balancing a fast reactiontime with the objective of preventing contamination with tungstic acid.It appears that an optimum pH is below 3.5, but is not less than 3.0.Near the end of the conversion, however, the pH of the APT mixture maybe allowed to rise to about 3.9-4.0. Allowing the pH to rise to about3.9-4.0 near the end of the conversion minimizes the amount of ammoniumhydroxide solution that may need to be added during the subsequentconcentrating step. During boiling the AMT solution for concentration,the pH becomes more acidic. Ammonium hydroxide solution may need to beadded to the boiling AMT solution to keep the pH of the AMT solutionfrom dropping below 3.0.

EXAMPLE 4

AMT was prepared from solid APT and water as described in Example 3.Individual batches were prepared at different times, and each batch ofAMT was analyzed for contaminant concentrations. After the analyses, thebatches were combined into a “Bulk Sample”. Elemental analyses for eachbatch and for the Bulk Sample are provided in Table 1. Atomic absorptionspectroscopy was used for detection of sodium, potassium, zinc, andmagnesium. Direct current plasma emission spectroscopy was used fordetection of the remaining listed elements.

TABLE 1 Bulk Batch 1 Batch 2 Batch 3 Batch 4 Batch 5 Sample Cr 0.00040.0004 0.0004 0.0004 0.0004 0.0004 Fe 00008 0.0017 0.0012 0.0010 0.00070.0010 Ni 0.0008 0.0008 0.0007 0.0004 0.0006 0.0008 Mo 0.0009 0.00070.0009 0.0009 0.0010 0.0010 Co 0.0007 0.0005 0.0004 0.0006 0.0004 0.0003Al 0.0005 0.0004 0.0004 0.0004 0.0004 0.0005 Cu 0.0001 0.0002 0.00010.0001 0.0001 0.0002 Si 0.0021 0.0029 0.0025 0.0016 0.0020 0.0015 Ca0.0003 0.0004 0.0004 0.0004 0.0003 0.0005 P 0.0013 0.0014 0.0014 0.00150.0027 0.0017 Na 0.0004 0.0007 0.0006 0.0007 0.0005 0.0004 K 0.00150.0008 0.0007 0.0005 0.0028 0.0012 Zn 0.0003 0.0003 0.0004 0.0004 0.00030.0004 Mg 0.0001 0.0002 0.0001 0.0001 0.0001 0.0001

EXAMPLE 5

A concentrated aqueous solution of the Bulk Sample from Example 4 wasspray dried. 10.5 L of the concentrated Bulk Sample AMT solutioncontained approximately 10.5 kg of AMT. The concentrated AMT solutionwas spray dried on a 3 foot NIRO® spray dryer. The inlet temperature ofthe spray dryer was held between 220° C. and 250° C., and the outlettemperature was held between 82° C. and 86° C.

After spray drying a portion of the concentrated AMT solution, the spraydried AMT powder was redissolved in double distilled water. Uponredissolution, the AMT solution was visually turbid. A turbiditymeasurement of the redissolved AMT solution yielded a value of 104,compared with a value of 32 for the concentrated AMT solution prior tospray drying. It was determined that the spray dryer was notsufficiently cleaned prior to spray drying the concentrated AMTsolution. Spray drying was halted, the spray drier was cleaned, and theprocess was resumed. Turbidity measurements of a solution of spray driedAMT collected after cleaning the spray drier indicated some continuingcontamination of the AMT, but the turbidity of the solution ofredissolved spay dried AMT gradually lessened to about 50, indicating areduced level of contamination.

EXAMPLE 6

Most of the spray dried AMT, as described in Example 5, was redissolvedin double distilled water, with a small portion being withheld foranalytical testing. The redissolved AMT solution was visually turbid.The solution was filtered. The filtration appeared to be effective asthe appearance of filtrate had a transparent amber color that wassimilar to concentrated AMT solution before spray drying. The turbiditymeasurements of the filtrate were comparable to the initial concentratedAMT solution. The filtered AMT solution was spray dried, and about 5.5kg of solid AMT was collected. Portions of the spray dried solid AMTwere dissolved in double distilled water and these solutions hadturbidity readings of about 30. An additional amount of about 1.7 kg ofmaterial was collected during cleanout of the spray drier. A solutionprepared from the cleanout material had turbidity reading of 43. Thecleanout material was stored separate from the 5.5 kg of spray dried AMTinitially collected to avoid any contamination with the higher puritysample. Several kilograms of AMT were lost during the cleanoutprocedure. The sprayed AMT powder had a water content of 1.8 wt. %.

Table 2 provides chemical analyses of the AMT before and after spraydrying. The values before and after spray drying vary little, indicatingthat the filtration removed the contaminants from the spray drying andthat the contaminants were not water soluble.

TABLE 2 Wt. % before Wt. % spray Wt. % after before Wt. % after Elementdrying spray drying Element spray drying spray drying Cr 0.0004 0.0004Si 0.0015 0.0014 Fe 0.0010 0.0016 Ca 0.0005 0.0005 Ni 0.0008 0.0010 P0.0017 0.0018 Mo 0.0010 0.0007 Na 0.0004 0.0007 Co 0.0003 0.0004 K0.0012 0.0013 Al 0.0005 0.0003 Zn 0.0004 0.0008 Cu 0.0002 0.0002 Mg0.0001 0.0001

Table 3 lists certain measured properties of the spray dried AMT powder

TABLE 3 Property Experimental AMT WO₃ Content  91.5% Ignition Loss (750°C.)   8.4% Heavy Metals (sum of Al, As, Cu, Fe, Mg, Mo, 0.0059% P, Pb,Zn) Non-Volatiles (sum of K, Na, Ca, and Si) 0.0043% pH of 30% aqueoussolution 2.85 Insolubles 0.0053%

FIG. 3 is a scanning electron micrograph (SEM) of a representativesample of the spray dried AMT. Spray drying the concentrated AMTsolution appears to have produced hollow spheres of AMT ranging from afew microns to about 60 microns in diameter. The larger spheres aremostly broken. No inorganic contaminants were detected by X-rayelemental analysis in either the 5.5 kg of AMT or the initially sprayed,visually contaminated powder.

EXAMPLE 7

3.3 L of water was heated to about 85° C. 1 kg of APT was added to thehot water and slurried. 638 mL of resin was added per kg of APT toadjust the pH between 3.0 and 3.5. The metatungstate ion is stable atthis pH. The slurry was heated to 85-95° C. to increase the APTsolubility and decrease the reaction time. Water was added to maintain aconstant concentration during the reaction process, which took about 2hours. After the reaction was complete, the AMT solution was separatedfrom the ion exchange resin by filtering, and concentrated viaevaporation. A high concentration of APT in the starting material isdesirable to increase throughput and minimize evaporation time.Preferably, all of the APT is reacted in the process due to thedifficulty of separating dissolved but unreacted APT. After sufficientwater was evaporated, the concentrated solution was filtered again toremove 0.5 μm to 1.0 μm particles and the pH was checked. The solid AMTwas dried.

EXAMPLE 8

Certain cation exchange resins used in the conversion of APT to AMTaccording to the present disclosure can be regenerated. The resin isseparated from the AMT solution and washed with a suitable acid toreplace ammonium ions bound to the resin particles with hydrogen ions.The capability to regenerate and reuse a cation exchange resin may beimportant given resin cost and the abrasive nature of thenon-solubilized APT particles present in reaction mixture during theconversion to AMT.

Scanning electron micrographs (SEM) were taken of (1) a sample ofas-received DOWEX® G-26 cation exchange resin (FIG. 4) and (2) the sameresin after an extended period of use in conversion of APT to AMT by themethod according to the present disclosure (FIG. 5). The SEM of FIG. 5shows that many of the resin beads had fractured, likely due to stirringof the reaction mixture and the abrasive nature of the undissolved APT.It is anticipated that gentler mixing, as would occur in a larger-scaleoperation, will result in less fracture of the resin beads. Fracture ofresin beads will increase the pressure necessary to pump wash water andregenerative acid through the resin. However, it was observed that usedfractured resin may still be regenerated and reused in the conversionmethod according to the present disclosure.

The DOWEX® G-26 cation exchange resin was successfully regenerated bysoaking the beads in 100-300% excess H₂SO₄ (75-200 mL concentrated H₂SO₄solution/Kg APT) for a suitable time, which is at least an hour but morepreferably 24 hours. The acid was then filtered from the resin using aBüchner funnel, and the resin was washed with double distilled water.The acid was neutralized and discarded.

It will be understood that the present description illustrates thoseaspects of the invention relevant to a clear understanding of theinvention. Certain aspects that would be apparent to those of ordinaryskill in the art and that, therefore, would not facilitate a betterunderstanding of the invention have not been presented in order tosimplify the present description. Although only a limited number ofembodiments of the present invention are necessarily described herein,one of ordinary skill in the art will, upon considering the foregoingdescription, recognize that many modifications and variations of theinvention may be employed. All such variations and modifications of theinvention are intended to be covered by the foregoing description andthe following claims.

1. A method, comprising: preparing an ammonium paratungstate mixturecomprising solid unheated stoichiometric ammonium paratungstate andwater; contacting the ammonium paratungstate mixture with a cationexchange material to lower a pH of the mixture to within a pH range atwhich a metatungstate ion is stable and an insoluble tungstic acid isnot formed; and maintaining the pH of the mixture within the pH rangefor a time period sufficient to convert substantially all of theammonium paratungstate mixture to a solution comprising ammoniummetatungstate.
 2. The method of claim 1, wherein the water comprises atleast one of deionized water, distilled water, and double distilledwater.
 3. The method of claim 1, wherein preparing an ammoniumparatungstate mixture comprises mixing the solid unheated ammoniumparatungstate with water having a temperature of 80° C. to 99° C.
 4. Themethod of claim 1, further comprising heating the ammonium paratungstatemixture to a temperature of 85° C. to 95° C.
 5. The method of claim 1,wherein contacting the ammonium paratungstate mixture with a cationexchange material comprises mixing the cation exchange material with theammonium paratungstate mixture.
 6. The method of claim 5, furthercomprising agitating the cation exchange material and the ammoniumparatungstate mixture.
 7. The method of claim 1, wherein the pH range is2.0 to 5.0.
 8. The method of claim 1, wherein the pH range is 3.0 to4.0.
 9. The method of claim 1, wherein the pH range is 3.0 to 3.5. 10.The method of claim 1, wherein the cation exchange material comprises acation exchange resin.
 11. The method of claim 10, wherein the cationexchange resin comprises a strong acid cation exchange resin.
 12. Themethod of claim 11, wherein the strong acid cation exchange resincomprises at least one of a compound including a sulfonic acid group anda compound including a phosphonic acid group.
 13. The method of claim 1,further comprising separating the cation exchange material from thesolution comprising ammonium metatungstate.
 14. The method of claim 13,wherein separating the cation exchange material from the solutioncomprising ammonium metatungstate comprises filtering the solutioncomprising ammonium metatungstate.
 15. The method of claim 14, furthercomprising concentrating the solution comprising ammonium metatungstateto form a concentrated ammonium metatungstate solution.
 16. The methodof claim 15, wherein concentrating the solution comprising ammoniummetatungstate comprises boiling the solution comprising ammoniummetatungstate.
 17. The method of claim 15, further comprising filteringthe concentrated ammonium metatungstate solution to remove at least aportion of insoluble residual particles from the concentrated ammoniummetatungstate solution.
 18. The method of claim 17, further comprisingdrying the concentrated ammonium metatungstate solution.
 19. The methodof claim 18, wherein drying the concentrated ammonium metatungstatesolution comprises spray drying the concentrated ammonium metatungstatesolution.
 20. A method for producing ammonium metatungstate fromammonium paratungstate, the method comprising: mixing solidstoichiometric unheated ammonium paratungstate and water having atemperature of 80° C. to 99° C. to provide an ammonium paratungstatemixture; combining an amount of strong acid cation exchange resin andthe ammonium paratungstate mixture to adjust a pH of the mixture to 3 to5; heating the ammonium paratungstate mixture to 80° C. to 100° C.;maintaining the pH of the ammonium paratungstate mixture at 3 to 5 untilsubstantially all of the ammonium paratungstate mixture is converted toan ammonium metatungstate solution; separating the cation exchange resinfrom the ammonium metatungstate solution; concentrating the ammoniummetatungstate solution to form a concentrated solution; filtering theconcentrated solution to remove at least a portion of insolubleparticles from the concentrated solution; and drying the concentratedsolution to obtain solid ammonium metatungstate.
 21. The method of claim20 wherein the water comprises deionized water, distilled water, ordouble distilled water.
 22. The method of claim 20, wherein: the amountof strong acid cation exchange resin adjusts the pH of the mixture to3.0 to 3.5; and the pH of the ammonium paratungstate mixture ismaintained at 3.0 to 3.5 until substantially all of the ammoniumparatungstate mixture is converted to an ammonium metatungstatesolution.